All closed refrigeration systems serially include a compressor, a condenser, an expansion device and an evaporator. Expansion devices include fixed orifices, capillaries, thermal and electronic expansion valves, turbines, and expander-compressors or expressors. In each of the expansion devices, high pressure liquid refrigerant is flashed as it goes through a pressure drop with at least some of the liquid refrigerant becoming a vapor causing an increase in specific volume. In an expressor, the volumetric increase is used to power a companion compressor which delivers high pressure refrigerant vapor to the discharge of the system compressor thereby increasing system capacity. Since the compression process occurring in the expressor is not powered by an electric motor, but by the flashing liquid refrigerant, overall refrigeration efficiency increases by the same amount as the system capacity.
For a typical applied pressure ratio for chillers, the pressure ratio, Pr, which represents the ratio of the discharge pressure to the suction pressure, is used to control the system. The volume ratio, Vi, is the ratio of the suction volume to discharge volume in the case of compression and the ratio of the discharge volume to suction volume in the case of expansion. For liquid expansion, the Vi, is on the order of ten, or more. For the same pressure ratio, the Vi for vapor expansion is only around three or four. The reason for the disparity between liquid and vapor expansion is that the volume of vapor is about eighty times that of the corresponding amount of liquid under the same conditions of temperature and pressure. Additionally, the phase change requires energy to convert the liquid to vapor. If an expander has a very high Vi, e.g. ten, or above, at the end of the inlet process liquid will fill the cavity defining the trapped volume of the expander. The expander will not be able to function properly in the absence of flashing, i.e. sub-cooled liquid, or if the flash rate does not match up with the volume change since liquids are not expandable. Prior art devices employ pre-throttling to significantly reduce the Vi, or Pr for the expander. Accordingly, at the end of the inlet process there are two phases inside the cavity volume. Pre-throttling wastes power in that no use is made of the energy.
A rotary vane or twin screw expander-compressor, or expressor, unit is used as an expansion device for achieving phase changing in air conditioning and refrigeration systems. The rotary vane, or twin screw, expressor is, effectively a two stage device with the expander being the first stage and providing the power for driving the compressor which is the second stage and which delivers compressed high pressure refrigerant to the discharge line extending from the system compressor to the condenser. According to the teachings of the present invention, liquid refrigerant is supplied to the inlet of the expander. At the end of the inlet process, high pressure vapor from the expressor compressor discharge is supplied to the trapped volume. This allows the expander to function properly while permitting the mechanical power of the liquid-to-vapor expansion to be fully derived. At start up some of the hot high pressure gas from the discharge line is supplied directly to the expander of the expressor which is thereby caused to start rotating.
It is an object of this invention to provide a high efficiency expansion of saturated or sub-cooled liquid to its vapor so as to derive mechanical power.
It is another object of this invention to control the rotational speed or flow capacity of an expresser.
It is an additional object of this invention to supply discharge gas directly to the expander of the expressor during start up.
It is a further object of this invention to eliminate the need for pre-throttling the liquid being supplied to the expander of an expressor. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.
Basically, saturated or sub-cooled liquid is supplied to the expander of an expressor. Starting just prior to the end of the inlet process or just after the completion of the inlet process, high pressure vapor from the expresser compressor discharge is supplied to the cavity defining a trapped volume under going expansion.